The usefulness of the relationship between lung cancer and the concentration of blood gastrin-releasing peptide (GRP) has been known as reported by also Yamaguchi et al. (see Patent document 1). However, GRP is a physiologically active substance and loses its activity in an extremely short period of time in serum or plasma. Therefore, it was difficult to practically use it as an immunoassay. Yamaguchi et al. found that the antigen stability sufficient to withstand the practical use as an immunoassay can be achieved by specifically assaying, among three types of pro-gastrin-releasing peptides (ProGRPs) which are precursors of GRP, an amino acid sequence of residues 31-98 (ProGRP 31-98) which does not contain a physiologically active region (an amino acid sequence of residues 1-27) and is a region common to the three types, and found that it is useful for diagnosis of lung cancer (see Patent document 1). According to this method, ProGRP can stably exist in serum or plasma equally up to 6 hours, therefore, it became possible to put it to practical use as an immunoassay method. Further, because an advantage of the use of plasma is not particularly obtained, serum which is generally used for assaying a cancer marker has come to be used as a sample.